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Day 25: November 7, 2011 Registers
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 25: November 7, 2011 Registers Penn ESE370 Fall Mehta & DeHon
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Previously… Clocking Discipline Follow discipline of combinational logic broken by registers Compute From state elements Through combinational logic To new values for state elements As long as clock cycle long enough, Will get correct behavior Penn ESE370 Fall Mehta & DeHon
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Today Clocking: Register Designs Dynamic Static
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Gate-Latch-Register Transistor Count? Total Transistor Width?
Capacitive load on data input? Capacitive load on clock? Penn ESE370 Fall Mehta & DeHon
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Alternate Registers Penn ESE370 Fall Mehta & DeHon
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How does this work as a register?
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Compare Gate-Latch-Register
Transistor Count? Total Transistor Width? Load on input? Load on Clock(s)? Penn ESE370 Fall Mehta & DeHon
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Weaknesses? Penn ESE370 Fall Mehta & DeHon
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Weaknesses Hold value on capacitance (“Dynamic”) Not drive to rail
Not actively driven Easily upset by noise Will leak away eventually Sets lower bound on clock frequency Cannot “gate off” clock when not in use Not drive to rail Less noise margin More static leakage – PMOS not completely off In 2010: Had good discussion on how long before leak away. Talked about quantifying magnitude… Penn ESE370 Fall Mehta & DeHon
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How Improve? Remember weaknesses: Holds value dynamically
Not driven to rail Penn ESE370 Fall Mehta & DeHon
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What is the difference? Penn ESE370 Fall Mehta & DeHon
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Transmission Gates Idea: use both NMOS/PMOS in parallel
NMOS passes the strong 0 PMOS passes the strong 1 Pass gates that swing full rail Often used in mux Penn ESE370 Fall Mehta & DeHon
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Transmission Gate Mux How is this different from a static mux? S S 1
In1 In2 /S 2 /S S How is this different from a static mux? Penn ESE370 Fall Mehta & DeHon
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How Improve? Remember weaknesses: Holds value dynamically
Not driven to rail Penn ESE370 Fall Mehta & DeHon
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Level Restorer (“Staticizer”)
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Without level restorer
This is just here to show contrast. Penn ESE370 Fall DeHon
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With level restorer Penn ESE370 Fall DeHon
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Level Restore What issue arises here? Penn ESE370 Fall DeHon
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Level Restore What issue arises here? Penn ESE370 Fall2010 -- DeHon
2010: it was tricky to get them to see the ratioed logic here. Penn ESE370 Fall DeHon
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Register with Level Restore
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Static Register Penn ESE370 Fall Mehta & DeHon
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Static Register Transistors? Total width? Clock load? Input load?
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Comparison Gate Latch Dynamic Latch Static Latch Area Large Small
Moderate Input Cap Delay Slow Fast Dynamic No Yes Full Rail Penn ESE370 Fall Mehta & DeHon
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Typical Static Register
Advantages: Static Full Rail Fast Phi0 Isolation inverters: Input Cap Input/Output Noise State Node Noise /Phi0 Penn ESE370 Fall Mehta & DeHon
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Admin Midterm Review tonight at 8pm (Moore 204)
Tuesday: Andre away (no office hour) Wednesday: Midterm No lecture Midterm 7-9pm in Towne 303 New Project out Thursday: <nothing> (read project) Friday: Class (Andre) Penn ESE370 Fall Mehta & DeHon
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Ideas Registers can be implemented more compactly with pass transistor-based designs Penn ESE370 Fall Mehta & DeHon
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